9,870 research outputs found
Ultrasonic transducer with Gaussian radial pressure distribution
An ultrasonic transducer that produces an output that is a symmetrical function comprises a piezoelectric crystal with several concentric ring electrodes on one side of the crystal. A resistor network applies different amplitudes of an ac source to each of the several electrodes. A plot of the different amplitudes from the outermost electrode to the innermost electrode is the first half of a Gaussian function. Consequently, the output of the crystal from the side opposite the electrodes has a Gaussian profile
Quantum Gravity and Non-unitarity in Black Hole Evaporation
We discuss the relevance of quantum gravitational corrections to the
functional Schr\"odinger equation for the information loss paradox in black
hole evaporation. These corrections are found from the Wheeler-DeWitt equation
through a semiclassical expansion scheme. The dominant contribution in the
final evaporation stage, when the black hole approaches the Planck regime, is a
term which explicitly violates unitarity in the non-gravitational sector. While
pure states remain pure, there is an increase in the degree of purity for
non-pure states in this sector. This result holds irrespective of whether full
quantum gravity respects unitarity or not.Comment: 6 pages, Latex, ZU-TH 25/9
Thoracic epidural anaesthesia impairs the hemodynamic response to acute pulmonary hypertension by depressing right ventricular contractility in pigs
The Fractality of the Hydrodynamic Modes of Diffusion
Transport by normal diffusion can be decomposed into the so-called
hydrodynamic modes which relax exponentially toward the equilibrium state. In
chaotic systems with two degrees of freedom, the fine scale structure of these
hydrodynamic modes is singular and fractal. We characterize them by their
Hausdorff dimension which is given in terms of Ruelle's topological pressure.
For long-wavelength modes, we derive a striking relation between the Hausdorff
dimension, the diffusion coefficient, and the positive Lyapunov exponent of the
system. This relation is tested numerically on two chaotic systems exhibiting
diffusion, both periodic Lorentz gases, one with hard repulsive forces, the
other with attractive, Yukawa forces. The agreement of the data with the theory
is excellent
Quantum Gravitational Collapse and Hawking Radiation in 2+1 Dimensions
We develop the canonical theory of gravitational collapse in 2+1 dimensions
with a negative cosmological constant and obtain exact solutions of the
Wheeler--DeWitt equation regularized on a lattice. We employ these solutions to
derive the Hawking radiation from black holes formed in all models of dust
collapse. We obtain an (approximate) Planck spectrum near the horizon
characterized by the Hawking temperature , where is the mass of a black hole that is presumed to form at the
center of the collapsing matter cloud and is the cosmological
constant. Our solutions to the Wheeler-DeWitt equation are exact, so we are
able to reliably compute the greybody factors that result from going beyond the
near horizon region.Comment: 27 pages, no figure
Canonical quantization of a particle near a black hole
We discuss the quantization of a particle near an extreme Reissner-Nordstrom
black hole in the canonical formalism. This model appears to be described by a
Hamiltonian with no well-defined ground state. This problem can be circumvented
by a redefinition of the Hamiltonian due to de Alfaro, Fubini and Furlan (DFF).
We show that the Hamiltonian with no ground state corresponds to a gauge in
which there is an obstruction at the boundary of spacetime requiring a
modification of the quantization rules. The redefinition of the Hamiltonian a
la DFF corresponds to a different choice of gauge. The latter is a good gauge
leading to standard quantization rules. Thus, the DFF trick is a consequence of
a standard gauge-fixing procedure in the case of black hole scattering.Comment: 13 pages, ReVTeX, no figure
Conformal Theory of M2, D3, M5 and `D1+D5' Branes
The bosonic actions for M2, D3 and M5 branes in their own d-dimensional
near-horizon background are given in a manifestly SO(p+1,2) x SO(d-p-1)
invariant form (p=2,3,5). These symmetries result from a breakdown of ISO(d,2)
(with d=10 for D3 and d=11 for M2 and M5) symmetry by the Wess-Zumino term and
constraints. The new brane actions, reduce after gauge-fixing and solving
constraints to (p+1) dimensional interacting field theories with a non-linearly
realized SO(p+1,2) conformal invariance. We also present an interacting
two-dimensional conformal field theory on a D-string in the near-horizon
geometry of a D1+D5 configuration.Comment: 32 pages, two figures, Latex. A version to appear in JHEP. A comment
is added on infinite dimensional Kac-Moody type symmetry of D1+D5 system
observed by Brandt, Gomis, Sim'o
The fractality of the relaxation modes in deterministic reaction-diffusion systems
In chaotic reaction-diffusion systems with two degrees of freedom, the modes
governing the exponential relaxation to the thermodynamic equilibrium present a
fractal structure which can be characterized by a Hausdorff dimension. For long
wavelength modes, this dimension is related to the Lyapunov exponent and to a
reactive diffusion coefficient. This relationship is tested numerically on a
reactive multibaker model and on a two-dimensional periodic reactive Lorentz
gas. The agreement with the theory is excellent
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